224 CHAELES R. STOCKARD 



known in the literature may be produced by one and the same experi- 

 mental treatment. For example, simply by lowering the surround- 

 ing temperature or by treating with a weak ether solution all 

 monsters may be produced. Secondly, the same structural abnor- 

 mality may be induced in the embryos of various species by a great 

 number of different experimental treatments. Thirdly, in all cases 

 the initial effect of the experimental treatment is a lowering of the 

 developmental rate, and the resulting deformity is always second- 

 arily due to this slow rate of development. Fourthly, the type of 

 monster or deforrnity is determined by the developmental period dur- 

 ing which the slowing in rate is experienced. An early slowing 

 will induce the growth of accessory embiyonic axes or duplicities, 

 while a similar reduction in rate at later periods may produce 

 anophthalmia or cyclopia, simple tubular brains, malformed otic 

 structures, deformities of the mouth or branchial arrangement, 

 etc., depending upon the time at which the rate of development 

 was slow. Slowing by a number of different ways if done at 

 the same developmental time will give closely similar defects. 



We may finally state a common law of both normal and abnor- 

 mal development as follows : Structural quality may be affected by 

 many things, but always depends directly upon the rate of develop- 

 ment of the part or of the individual. I have many reasons for 

 believing that this law equally applies during postnatal growth 

 and change in higher animals, as well as during their prenatal 

 development. 



In a study of a number of different embryos it will be observed 

 that a particular structural modification is more common in 

 one species of egg than in another. With trout eggs for example, 

 duplicities more readily occur than in the egg of Fundulus. While, 

 on the other hand, cyclopia and certain eye defects are more 

 common among deformed specimens of Fundulus than among 

 those of the trout. It would seem as though particular moments 

 and localities were more susceptible to modifications in one egg 

 as a result of slow development than in another. Further, cer- 

 tain eggs are in general much more sensitive than others and, as 

 is well known, more frequently deformed. Their developmental 

 rate is less strongly regulated than is the case in the more resist- 



